Just noticed that my last note had been posted already; sorry. In addition, I'm wondering whether a plaintext/cyphertext pair is considered to have a one-to-one isomorphism; in particular, in the case where you have a plaintext/cyphertext pair but no knowledge of the method of encryption, is there a one-to-one isomorphism? In the case of a one-time pad, is there isomorphism, and if so, of what sort? Thank you. ---- Robert W. Clark Just Say No! to the rclark@nyx.cs.du.edu Big Brother Chip